20 February 2017

Which patch? Dispatch.

**Authors note: I started writing this post a year ago. Hence the references are a bit dated, but the content is relevant.

At the behest of the goodpeopleof reddit, I figured I would talk a little bit about a recent issue that cropped up in Planetside 2: a runaway thread in our threading library that went mostly unnoticed and caused reduced performance.

Threads Are Hard

Writing good multi-threaded code isn't easy. I challenge the linked article's author in that multi-threaded programming is actually difficult. Sure, some of it boils down to programmers not following best practices, but there are several other facets that you don't encounter in single-threaded programming:

Statistical testing (problems occurring with a low enough frequency that you don't see them until they reach the Live environment)

Woody shares my expression.

However, the availability of hardware is favoring power by having increasingly larger numbers of cores. To take advantage of that power, you need threads.

Concurrency

Threads are a solution to the problem of trying to make a computer seem to do multiple things at the same time. In the olden days, threads didn't exist, but systems could start other processes by forking the process. This would create a copy of the process that could do different things without affecting the parent process. By being a copy, changes that one process made to its memory and variables wouldn't affect the other process. The two processes would still be able to communicate somewhat via IPC, but this is generally much slower than, say, setting a variable within a process.

Like my threads?

But sometimes you want one process to be able to do multiple things at the same time. That's where threads come in to play. Since threads allow multiple things to happen "simultaneously" within the same process, you have concurrency.

I put "simultaneously" in quotes, because multi-processor systems of yore were generally limited to server-class hardware. It was uncommon to find a user's home machine with more than one processor. This means that the system could really only do one thing at a time, but it looked like it was doing things simultaneously because it was switching threads (i.e. things that it was doing) very, very quickly. These days, everything is multi-processor. My house thermostat is probably multi-CPU (ok, not really). The focus in hardware shifted from doing one thing very fast (higher clock speed aka GHz on CPUs) to doing many things pretty fast at the same time. The previous generation consoles (Playstation 3 and Xbox 360) had three to four CPUs whereas today's console generation (PlayStation 4 and Xbox One) have eight. My work computer has 12 "logical" cores.

Early threading involved creating threads for very specific tasks. For example, EverQuest II largely runs in a single thread, but creates specific threads for loading files, talking to the streaming asset server, updating particle effects, etc. Most of the time those specific threads are doing nothing; they're just sleeping. As the number of processors in a system grew, it becomes less practical to have dedicated, specific threads, especially when the number of processors in a system differs from system to system.

Synchronization

Let's take a break for a second to talk about a related topic. Synchronization is a big, huge, gargantuan topic wherein lies most of the problems with multi-threading. I'm only going to say a few words about Synchronization.

Yep, like that.

Nearly everything in the computer system can be considered a resource that must be shared: files, memory, CPU time, DVD drives, graphics, etc. What must be shared must be synchronized so that separate threads don't counter-productively stomp on each other. Process memory is probably the most often shared and problematic resource because everything interacts with it. Something like a file is fairly easy to synchronize because nearly every access of it requires a function call. Memory, on the other hand... For instance, here is an example of a function that just increments a number. What would the value be if you had two threads on a multi-processor machine calling this function 1000 times?

Hint: it's usually not 2000. Surprise! Incrementing a number is not an atomic operation, it's actually a read-modify-write operation. This is one of those things that makes multi-threaded programming so hard! To protect the section of code that increments the number, you have to use some sort of synchronization primitive, like an atomic intrinsic, mutex, spin-lock, or the like.

Tasking

When you have more processors available, it makes more sense to break problems down in to logical tasks or units of work. Instead of having a dedicated thread to load files, now you just have a "load file" task. You have a "collect garbage" task. You have an "animate entity" task. Task, task, task.

Khaaaaaaaannn!!!

This concept of tasks helps you to fill all available processors with work to do. Theoretically, if you can keep the task backlog full, CPUs will always have work to do. If you're using 100% of the available processors, you're doing the maximum amount of work that the system can do.

Dispatch

In 2009, Apple launched Mac OS X 10.6 with a programming API (libdispatch) marketed with the name Grand Central Dispatch (GCD). It is, among other things, a generic task execution system. You have a function or a task that you want to run in the main thread or a separate thread at specific priority levels, immediately or at a scheduled time in the future. You just take that function or task and throw it on a "dispatch queue" and let it run. Simple. Powerful. Efficient. I quickly fell in love with what GCD could offer when I used it for my iPhone game, Bust a Mine.

Fast forward to mid-2014. I became Technical Director of Planetside 2. The team was working on porting Planetside 2 to the PlayStation 4. Performance profiling was showing that the CPUs on the PS4 were slower than average CPUs on our PC players' machines, and Planetside 2 was still largely single-threaded. We started looking at threading technologies like Intel's Threading Building Blocks, OpenMP, and even the C++11 thread support library. However, given my experience with libdispatch and the approach of looking at the problem as tasks rather than dedicated threads, we decided to look around for something similar. We found xdispatch, a port of libdispatch to Windows and Linux (libdispatch was originally written for Mac OS X which is based on BSD). However, it had some issues: namely it didn't support the PS4 (few things did) and was based on a much older version of libdispatch. We began adapting it to the PS4 and it gave us a solid framework to start multi-threading Planetside 2.

Adaptive Tuning

We developed a threading sub-team on the PS4-on-PS2 project that had a primary requirement (increase performance) through two primary points of attack: 1) tune xdispatch to do what we needed and 2) adapt existing threads and operations into tasks that could be done concurrently. Ideally these changes would carry over to the PC version of the game as well.

Both of these facets were challenging. On the tuning front we discovered that part of the reason that GCD works so well on Mac OS X is because the kernel--the core of the operating system itself--is controlling the dispatch scheduling. We didn't have that ability on the PS4, nor could we get the information about how busy the system is! We went several iterations on how to deal with this, but eventually settled on working around this by setting up some guidelines--the standard dispatch queues would only be used for CPU-intensive work (calculation, animation, decompression, etc.) and we would limit them to the number of CPUs available.

The second facet was a much longer pole that would continue throughout the project. Converting existing dedicated threads and PS2's old job-queue system to Dispatch was easy and quickly done, but that didn't net nearly the performance that we would need to see to be viable on PS4. We would have to go much deeper. This would involve taking core aspects of PS2's main loop and breaking them into tasks--entity processing, physics, animation, rendering, etc. This is difficult to do with C++ because non-thread-safe side-effects are nearly impossible to find; we would have to identify everything non-thread-safe that was happening in the single-threaded code before converting it to tasks.

OMG Bugs

That was the song, right?

The reddit post that originally spawned this blog post sheds some insight on to a problem that still existed a year after the PS4 version originally launched. Namely, PC players identified that a thread would take a whole CPU, but they found that if they killed it, performance got better and nothing bad really happened (or was not immediately visible). This was found to be a bug in timers in xdispatch that, to my knowledge, still exists. You can read the above link for more technical information, but it had to do with bad assumptions in the PC port of software originally written for BSD. Shockingly, the problem also existed in the PS4 build even though it shouldn't have. It looks like the timer implementation in xdispatch (and the libdispatch version that it was based on) was functional but not very efficient, so we wrote a new timer outside of xdispatch and used that instead.

Still later, we finally got a handle on one of our long-standing (post-PS4-launch) crash bugs. This was a crash bug that we had never seen internally (see my above point about bugs becoming a statistical problem). It looked like a memory corruption problem, which just made all of the programmers reading this shudder in horror. Memory corruption is terrible. It is an evil problem and few good tools exist to locate it assuming you can figure out how to make it happen. But find it we did, and it was also an issue with converting systems to multi-threading. In this case, an 'animation complete' flag was in a data structure that was getting freed before the task performing the operation finished and set the flag. Since the memory was freed before the flag was set, sometimes the memory had been reused for other things, hence the corruption. This was a problem not with Dispatch itself, but with how a previously single-threaded operation had been converted to a task.

Most recently, we began hearing reports of 'lock-ups' and 'hangs' from PS4 players. This coincided with an update to PS4 SDK 3.500 (from 2.000) for Planetside 2, which, among other things, gave us an additional half of a CPU to use for game logic (with 2.000 the system would reserve two whole CPUs for itself, whereas after 3.000 it only used one-and-a-half). Because of this, we ramped up Dispatch (now a completely retooled version no longer based at all on xdispatch but based on a more current version of libdispatch) to take advantage of that half-a-CPU. Eventually we determined this to be the cause of the lockup, but for unexpected reasons. The game was not experiencing a dead-lock, but a form of live-lock; all of the CPUs were running threads, they just weren't making any progress. This was because of a degenerate case between the design of libdispatch and the PS4 scheduler. Basically, the internals of libdispatch (and our Dispatch library that was based on it) are lock-less--they are doing atomic operations rather than locking mutexes. Some of these atomic operations are loops waiting for conditions to be met, or multiple compare-and-exchange operations in a loop; they try to do an operation based on old information and retry with updated information if it fails. But the PS4 scheduler will not run any thread of a lower priority if a higher priority thread is runnable. We could end up in a state where a lower-priority thread would be preempted after it changed conditions for a higher-priority thread. This would cause a sort of priority inversion that would never resolve. Most operating systems will at least give some time to lower priority threads to prevent starvation, but the PS4 does not. Indeed the default scheduler for the PS4 is the FIFO scheduler, but even the round-robin scheduler will not run lower-priority threads. Our solution to this involved applying a progressive algorithm that would eventually put threads to sleep in extreme cases in order to allow the live-lock to resolve. Generally this might look like a slight momentary dip in frame rate or may not even be noticed at all.

Looking Forward

Make sure your blinker is on.

We're always looking for ways to increase performance across all platforms. As other Daybreak games ramp up we're finding new ways to eke out increasing frame-rates and sharing that knowledge among the teams. Our internal Dispatch implementation is moving into other Daybreak titles and future projects, and it all started here, on Planetside 2. Efforts have been made to keep these types of changes in parity between the different games.

1 comment:

Forces a read for every access so the compiler will not look at the register that contains the old value, nor will it decide to optimize out the code as it never changes.

Using two volatile variables both reader and writer can use them as a mutex without the system overhead of same. That isn't always necessary if the threads communicate with pipes.

Also, I normally allocate memory from the outer-most process. Then pass a pointer to the structure instead of using globals. If two processes need to communicate then a message (or some such) is written to the blocked reader.

I learned most of my OS work on the job. We wrote a real-time kernel that ran on unprotected systems. No write protected memory, if you wanted it write protected we burned it into EPROMs. Took a board full of EPROMs to test the kernel in a system.